1. Field of the Invention
The invention relates to a sheet feeding apparatus that is provided in an image forming apparatus such as a copying machine, a facsimile machine, a laser beam printer, or a complex machine and an image forming apparatus provided with the sheet feeding apparatus.
2. Description of the Related Art
The sheet feeding apparatus provided in the image forming apparatus is required to feed sheets at a constant interval between sheets so as to improve productivity (the number of sheets to be formed with images per unit time).
In a separation method of the related art using a retard roller which rotates in a direction opposite to a sheet feeding direction, the following configuration has been proposed to improve the productivity.
The separation method using the retard roller is configured such that sheets are sent by a pickup roller and then is separated and fed one by one by a feed roller and a retard roller. Since a rotary driving is transmitted to the retard roller in the direction opposite to the sheet feeding direction through a torque limiter, when a plurality of sheets enters the separation nip portion between the feed roller and the retard roller, the retard roller reversely rotates to separate the sheets one by one. In addition, when one sheet is fed to the separation nip portion between the feed roller and the retard roller, the retard roller is driven by the sheet to be fed and then rotates in a sheet sending direction.
In the separation method using the retard roller, then, when the retard roller is driven by the sheet, the driven rotation of the retard roller is transmitted to the pickup roller to rotate the pickup roller, in order to improve the productivity. Therefore, the pickup roller feeds the next sheet successively after a trailing end of a preceding sheet passes through the position of the pickup roller so that the leading end of the next sheet is moved to the position of the separation nip portion (see U.S. Pat. No. 8,430,393).
The separation method using the retard roller disclosed in U.S. Pat. No. 8,430,393 can resolve variation of the leading position (interval between sheets) of the sheet to be fed in a successive sheet feeding operation. In this technique, that is, when the retard roller is rotating in the sheet feeding direction, the rotation is transmitted to the pickup roller to rotate the sheet in the sheet sending direction. In addition, when the retard roller is rotating or stopping in the direction opposite to the sheet feeding direction, the pickup roller does not feed the sheet. Thus, during the successive feeding, the pickup roller continuously rotates in a state where the retard roller is driven by the preceding sheet. Therefore, the next sheet is successively sent without a gap with the preceding sheet, and then the leading end of the next sheet is stopped at the separation nip portion between the feed roller and the retard roller. Accordingly, during the successive sheet feeding, the leading end of the sheet is always positioned at the separation nip portion, and thus the interval between the sheets can be constant. Thus, the productivity can be improved.
However, since the technique disclosed in U.S. Pat. No. 8,430,393 is a method of interlocking the pickup roller with the rotation of the retard roller, the load when the pickup roller sends the next sheet during the interlocking acts to the retard roller. That is, the load is a reaction force which is applied to the pickup roller from the sheet, and an unnecessary force is applied to the retard roller through an interlocking mechanism by the load. This may cause the decrease in a pressing force (separation force) against the sheet of the retard roller to occur a slip between the retard roller and the sheet, and thus there is a possibility to make a trouble of a defective sheet feeding or an early abrasion of the retard roller.
The invention is to provide a sheet feeding apparatus and an image forming apparatus which are configured so as not to decrease the separation pressure by the load from the pickup roller in a way where the pickup roller interlocks with the rotation of the retard roller.